1. Field of the Invention
This invention relates to an optical disk recording/reproducing method, optical disk recording/reproducing apparatus and optical disk and more particularly to parameter adjustment at the recording time of a signal on an optical disk and at the reproducing time of a signal from the optical disk.
2. Description of the Related Art
When information is recorded on an optical disk, a laser beam is used. At this time, it is required to optimize the recording power of the laser beam. As the technique for optimizing the recording power, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. H4-141827 is provided.
In Jpn. Pat. Appln. KOKAI Publication No. H4-141827, information is recorded and the amplitude of a reproduction signal obtained at this time is measured while the recording power is gradually changed. An optimum value of the recording power is derived based on the relation between the recording power and the amplitude of the reproduction signal. Further, information is recorded and the bit error rate obtained at this time is measured while the recording power is gradually changed. An optimum value of the recording power is derived based on the relation between the recording power and the bit error rate. The recording waveform of an electrical signal to generate a laser beam takes a so-called multi-pulse form which is repeatedly set at high and low levels in a short period of time. If the laser beam obtained by use of the recording waveform is applied to a track of the optical disk, marks are formed. Data contents are expressed by the widths of marks and spaces arranged on the track.
As the technique using the relation between the first and last pulses of the recording wave of the multi-pulse form, the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2000-149262 is provided. In this case, the first pulse of the recording wave of the multi-pulse form is called the first pulse and the last pulse thereof is called the last pulse. In Jpn. Pat. Appln. KOKAI Publication No. 2000-149262, the recording waveform is divided into pattern groups by using the length of each mark to be recorded and the length of a space preceding (or succeeding to) the mark as a pair and the width of the first pulse (or last pulse) is optimized for each pattern group. Information is recorded by changing the width of the first pulse (or last pulse) and the amount of jitter occurring at this time is measured. The jitter amount is a variation in time at which the reproduction signal passes through a slice level set in a level slicer. The optimum value of the width of the first pulse (or last pulse) is derived based on the relation between the width of the first pulse (or last pulse) and the jitter amount.
As a system for reproducing information recorded on the optical disk, a slice and identification system and PRML (Partial Response and Maximum Likelihood) identification system are provided.
Simply speaking, the slice and identification system converts reflection light reflected from the optical disk into an electrical signal by use of the photoelectric converter of a pickup head. The electrical signal is sliced in the slice circuit and converted into decoded binary data.
The PRML identification system is as follows. In the PRML identification system, a PR (Partial Response) characteristic corresponding to the recording/reproducing characteristic is used. As an example, a PR(1, 2, 2, 2, 1) characteristic is explained. The PR(1, 2, 2, 2, 1) characteristic indicates a characteristic in which a reproduction signal corresponding to a code bit “1” is set to “12221”. A reproduction signal is obtained by the convolution operation for the code bit series and the series of “12221” indicating the PR characteristic. For example, a reproduction signal for the code bit series “0100000000” is set to “0122210000”. Likewise, a reproduction signal for the code bit series “0110000000” is set to “0134431000”, a reproduction signal for the code bit series “0111000000” is set to “0135653100”, a reproduction signal for the code bit series “0111100000” is set to “00135775310” and a reproduction signal for the code bit series “0111110000” is set to “0135787531”. In the PR(1,2,2,2,1) characteristic, the reproduction signal is set to nine levels. The reproduction signal calculated by the convolution operation is an ideal reproduction signal (which is hereinafter referred to as a pass). However, in the actual reproduction signal, the characteristic is not always exactly set to the PR(1,2,2,2,1) characteristic and the reproduction signal contains deterioration factors such as noises. In the PRML identification system, the characteristic of a reproduction signal is set closer to the PR characteristic by use of an equalizer. The reproduction signal with the characteristic set closer to the PR characteristic is called an equalized reproduction signal. After this, a pass having the minimum Euclidean distance with respect to the equalized reproduction signal is selected by use of a Viterbi decoder. The pass and code bit series are set in a one-to-one correspondence. The Viterbi decoder outputs a code bit series corresponding to the selected pass as decoded binary data.
Recently, with an increase in the density of the optical disk, the PRML identification system is more frequently used instead of the slice and identification system.
In the optical disk system using the PRML identification system, it is assumed that the reproduction signal is not a binary signal but a signal with a three or more values or a so-called multi-value signal. The amplitude of the reproduction signal is a difference in level between the maximum and minimum values of the reproduction signal. A method for optimizing the recording waveform based on the measured value of the amplitude of the reproduction signal is a method based on the assumption that the reproduction signal is a binary signal. That is, the recording waveform is different from the recording waveform optimum for the optical disk system using the PRML identification system.
In the optical disk system using the PRML identification system, a reduction in the jitter amount does not always lead to enhancement of the quality of the reproduction signal. That is, the recording waveform cannot always be optimized by using the measurements of the jitter amount.
In the measurement of the bit error rate, measured values greatly vary due to local defects of the optical disk. Therefore, the recording waveform cannot be sufficiently optimized by use of the method for optimizing the recording waveform based on the measured values of the bit error rate because of an influence by a variation in the bit error rate. Further, in order to measure the bit error rate, it is necessary to record/reproduce an extremely long code bit series on the optical disk. Therefore, the method for optimizing the recording waveform based on the measured values of the bit error rate requires a long time for optimizing the recording waveform.